1. Field of the Invention
The present invention relates to a magnetic sheet, which is capable of suppressing undesirable electromagnetic waves emitted from an electronic equipment, and inhibiting electromagnetic disorders occurred in an electronic equipment.
2. Description of the Related Art
The magnetic sheets are used for depression of noise, or for RFID. For the purpose of depression of noise, along with rapid developments of electronic equipments, e.g. personal computers and mobile phones, such as down sizing and higher frequency, these electronic equipments suffer from a noise interference due to outer electromagnetic waves and interference of noises occurred within the electronic equipment. In order to suppress such interferences, various countermeasures have been taken for the noises. For example, a magnetic sheet (noise depressing sheet) is disposed adjacent to a noise emitting source or a noise receiving source.
The magnetic sheet is formed by adding alloy (magnetic powder) such as Fe—Si—Al to an epoxy resin, acrylic resin, or the like, and then being cured to form into a sheet by hot pressing. The magnetic powder inhibits noise, and functions as a so-called noise depressor. The better the effect of inhibiting the noise of the magnetic sheet is, the larger an imaginary part μ″ of the magnetic permeability of the magnetic sheet is.
On the other hand, in the case where the magnetic sheets are used for RFID, as a recent trend in the art, radio communication using a coil antenna in an electromagnetic induction method, which is represented as mobile information terminals having IC tags referred as Radio Frequency Identification (RFID), has been widely used. For example, in the mobile information terminal, various conductors (metals) such as a metal casing or metal parts are disposed adjacent to an antenna element for transmitting and receiving due to downsizing of the mobile information terminal. In this case, a magnetic field usable for the transmission is largely attenuated because of the metal present adjacent to the antenna element, and as a result, a communication distance for RFID transmission in an electromagnetic induction system may be shortened, or transmission or receiving of a radio frequency may become difficult because a resonance frequency is shifted. To prevent these electromagnetic disorders, a magnetic sheet is placed between the antenna element and the conductor. As for a function of RFID, it is preferred that the magnetic sheet has a large real part μ′ of the magnetic permeability, and a small imaginary part μ″ of the magnetic permeability.
As the magnetic sheets are mounted in electronic equipments or the like, the magnetic sheets are required to have heat resistance and fire retardance. Therefore, a fire retardant is generally contained in the magnetic sheets.
However, as a conventional fire retardant, a halogen-based compound typified by a bromine-based fire retardant is used. The halogen-based compound has a large environmental load, as it generates a harmful material such as environmental hormone upon burning. Thus, the use of the halogen-based compound tends to be restrained. On the other hand, a phosphorous-based fire retardant has also been used as the fire retardant, but it is inferior to the halogen-based fire retardant in terms of the properties as the fire retardant.
In current situation, there are clients who desire a magnetic sheet having flexibility, and clients who desire a magnetic sheet having a certain hardness or harder to improve workability.
As halogen-free magnetic sheets each containing a fire retardant, the following magnetic sheets are known in the art: magnetic sheets containing aluminum hydroxide, magnesium hydroxide, melamine, or the like as a fire retardant, and red phosphorus, ammonium polyphosphate, or the like as a flame-retardant auxiliary (see Japanese Patent Application Laid-Open (JP-A) No. 2003-324299); magnetic sheets containing as a fire retardant and/or flame-retardant auxiliary, melamine, melamine derivatives, red phosphorus, ammonium polyphosphate, pentaerythritol, dextrin, polyvinyl acetate, or the like (see JP-A No. 2003-243879); and magnetic sheets containing as a fire retardant, aluminum hydroxide or magnesium hydroxide together with red phosphorus (see JP-A No. 2004-71993).
However, these fire retardants and/or flame-retardant auxiliaries do not have sufficient fire retardance, and when they are added in a large amount for obtaining high fire retardance, the magnetic permeability of the magnetic sheet is decreased. Particularly, in the case where a halogen-free fire retardant is used, generally if a large amount of the fire retardant is not added, the fire retardance cannot be sufficiently exhibited. However, when the large amount of the fire retardant is added to give the fire retardance to the magnetic sheet, the magnetic sheet may be hardened. Thus, in order to obtain a soft magnetic sheet, the amount of the fire retardant needs to be decreased as small as possible, at the same time as obtaining sufficient fire retardance.
Moreover, the magnetic sheet has poor humidity resistance, and the fire retardant bleeds on a surface of the magnetic sheet, or the magnetic sheet varies in its thickness and magnetic properties by absorbing water. The magnetic sheet is required to have higher reliability for use in the electronic equipments, such as mobile phones, personal computers, and the like.
In the case where a soft magnetic sheet is produced, a large amount of magnetic powder is added to obtain high magnetic permeability, but causing to form a hard magnetic sheet.